In the recent drive for higher integration densities and operating speeds in LSI devices, the pattern rule is made drastically finer. The background supporting such a rapid advance is a reduced wavelength of the light source for exposure. The change-over from i-line (365 nm) of a mercury lamp to shorter wavelength KrF excimer laser (248 nm) enabled mass-scale production of dynamic random access memories (DRAM) with an integration degree of 64 MB (processing feature size ≦0.25 μm). To establish the micropatterning technology necessary for the fabrication of DRAM with an integration degree of 256 MB and 1 GB or more, the lithography using ArF excimer laser (193 nm) is under active investigation. The ArF excimer laser lithography, combined with a high NA lens (NA≧0.9), enables mass-scale fabrication of 65-nm node devices. For the fabrication of next 45-nm node devices, the F2 laser lithography of 157 nm wavelength became a candidate. However, because of many problems including a cost and a shortage of resist performance, the employment of F2 lithography was postponed. ArF immersion lithography was proposed as a substitute for the F2 lithography (see Proc. SPIE Vol. 4690, xxix, 2002). Development works are currently concentrated thereon.
In the ArF immersion lithography, the space between the projection lens and the wafer is filled with water and ArF excimer laser is irradiated through the water. Since water has a refractive index of 1.44 at 193 nm, pattern formation is possible even using a lens with NA of 1.0 or greater. The theoretically possible maximum NA is 1.44. The resolution is improved by an increment of NA. A combination of a lens having NA of at least 1.2 with strong super-resolution technology suggests a way to the 45-nm node (see Proc. SPIE Vol. 5040, p 724, 2003).
The ArF immersion lithography wherein exposure is made in the presence of water on a resist film has a possibility that an acid generated in the resist film and a basic compound previously added to the resist material can be, in part, leached in immersion water. As a result, pattern profile changes and pattern collapse can occur. It is also pointed out that water droplets remaining on the resist film after scanning, though in a minute volume, can penetrate into the resist film to generate defects. It was then proposed to provide a protective coating between the resist film and water to overcome these drawbacks (see 2nd Immersion Workshop Resist and Cover Material Investigation for Immersion Lithography, 2003).
Among fluorinated protective film materials, protective films made of perfluoroalkyl compounds use fluorocarbons like Freon® as the diluent for controlling a film thickness and as the stripper for stripping off the protective film after exposure. As is well known, the use of fluorocarbons is a consideration in view of environmental protection. In addition, special units for coating and stripping of the protective film must be added to the existing system. These factors raise serious problems on practical use.
One proposal for mitigating practical drawbacks of the above protective film is a protective film of the type which is soluble in alkaline developer (JP-A 2005-264131). The alkali-soluble protective film is epoch-making in that it eliminates a need for a stripping step or a special stripping unit because it can be stripped off at the same time as the development of a photoresist film. However, there is still left a room for improvement at a practical level because the solvent capable of dissolving the photoresist cannot be selected as the solvent for diluting the protective film material for coating, and a special unit for coating the protective film is necessary.
JP-A 2006-048029 discloses the addition of a hydrophobic fluorinated compound to resist material as the means for inhibiting water from penetrating into a resist film. This method is advantageous over the use of a resist protective film because the steps of forming and removing the protective film are unnecessary. However, when a hydrophobic fluorinated compound is added to a resist material, the resulting resist film on its surface has an increased contact angle, especially after development, tending to form defects known as “blob defects.” It is then desired to have a resist additive which serves to reduce the contact angle of a resist film as developed while maintaining effective water barrier properties and for further performance improvements, to have a novel monomer that reflects such a design concept and whose structure can be tailored so as to comply with any particular performance required.